Modern wind turbines usually comprise a rotor with a considerable diameter and width. Mounting a wind turbine could include the steps of transporting the different elements to the site of the wind turbine, assembling the tower sections and the tower, lifting the wind turbine nacelle with a crane and mounting the nacelle on the top of the tower, assembling the wind turbine rotor on the ground, lifting the wind turbine rotor with a crane and mounting the rotor to a low speed shaft extending from the nacelle.
The usual way of mounting a wind turbine comprises a number of drawbacks which have become more and more severe with the increasing size and width of the wind turbine rotor. Assembling the wind turbine rotor on the ground is especially difficult as it requires a large area free of obstacles which is substantially horizontal and stable in order to be accessible for the assembly workers and the crane. Furthermore, lifting the rotor to the nacelle is rather complicated as the rotor must be turned by 90° in midair.
From US 2005/019166 A1, it is known to mount a rotor hub to which two blades are already mounted to the nacelle and then mounting the remaining rotor blade to the rotor hub with the blade in a vertical position.
In other lifting systems it is known to pre-mount the wind turbine hub on the nacelle and then lift each wind turbine blade individually to a position next to the hub and perform the mounting of the blades. In one such system, which is disclosed in US 2006/0120809 A1, the lifting is performed with the wind turbine blades held vertically with a lifting device. However, in this lifting system the blade has to be vertical during lifting and mounting. This means that, as a part of the lifting process, the blade has to be turned and during the positioning of the blade there is little control over the angular blade orientation.
In another lifting system which is disclosed in US 2006/0147308 A1, the blade is held substantially horizontal, resting in slings which are held by wires fixed to the turbine blade. This system has the advantage that the blade axis can be maintained in the same position during lifting and mounting as when resting on the ground. However, any wind occurring at the time of lifting will tend to deflect the blade. Consequently, such lifting requires a number of persons stationed at some distance from the turbine during lifting and holding long ropes to help steer the blade. For large blades and tall towers the control of such steering becomes a major challenge.